1. Field of the Invention
This invention relates to the fabrication of electronic components and in particular the quality control of such fabrication.
2. Art Background
Generally, after integrated circuits and other electronic devices, such as discrete field effect transistors are produced, they are encapsulated in materials such as ceramics, metals and plastics. This encapsulation protects the device from the atmosphere and from abrasion. However, during the encapsulation it is almost inevitable that some water vapor is trapped between the encapsulant and the encapsulated device. If an excessive amount of water is trapped, the device generally will degrade to an unacceptable level through mechanisms such as corrosion or inducement of abnormal leakage currents in the device. Therefore during device manufacture, one quality control procedure is to measure the entrapped water in a representative sampling of devices from a batch of devices to ensure that this representative group of one or more devices, and by inference the entire batch, does not have an excessive amount of trapped water.
Generally a large number of techniques are available for detecting water vapor in continuous fluid flow and large gas volumes. flow and large gas volumes. (See Wexler, Arnold, ed., "Humidity and Moisture: Measurements and Control In Science & Industry," Reinhold Publishing Corp., (New York, 1965) for a review of available measurement methods including infrared techniques.) However, for device quality control purposes involving small encapsulated volumes, the conventional method of measuring water vapor levels employs mass spectrometry. In this technique, the encapsulant layer of the sample device is punctured in a vacuum chamber to release the trapped water vapor. An inlet to a mass spectrometer such as a quadrupole spectrometer is positioned in proximity to the punctured area of the encapsulated device. The released water vapor is thus sampled and a measure of the water vapor is made by integrating the amount of water detected over a period of time. Typically, on a mass spectrometric apparatus, the time period over which the amount of detected water vapor is integrated is arbitrarily chosen. (See R. W. Thomas, "Moisture, Myths, and Microcircuits," IEEE Transactions On Parts, Hybrids and Packaging, PHP-12, pp. 167-171 (1976) for a review of mass spectrometric quality control measures.)
Although the mass spectrometric method is widely practiced, the results obtained are not entirely satisfactory. The difficulties in mass spectrometric analysis are emphasized by the numerous literature references indicating that tremendous care is required to maintain consistent results for such mass spectrometric measuring of water vapor content in devices. Indeed, studies have indicated that the correlation between measurements of controlled samples on different equipment have yielded a large scatter in the measured values of the water vapor. (See R. W. Thomas and D. E. Meyer, "Moisture In SC Packages," Solid State Technology, 17, pp. 56-59 (1974) and K. L. Perkins, "Moisture Measurement Studies," Semiconductor Measurement Techniques and Reliability Techniques for Cardiac Pacemakers, pp. 60-65 NBS Special Publication 400-50, US Government Printing Office (1978).) Thus, two devices which potentially can undergo substantially different levels of degradation may well be incorrectly analyzed by conventional techniques to be both within acceptable quality control levels.